It is well known to use activated materials such as activated charcoal in an air stream in an attempt to remove odors. Conventional carbon adsorbents have been found to have a broad range of effectiveness against odors. However, such activated carbon adsorptive techniques of removing odor from an air stream are effective only for a relatively short time period, are relatively inefficient at low odor concentration levels, have performance which declines rapidly at higher relative humidities, and frequently result in a relatively high pressure differential across the activated carbon filter media. Odors are adsorbed onto and into the pores of the charcoal. These adsorbed odors are desorbed to varying degrees by such conditions as temperature changes, the super saturation of the adsorptive surfaces, and by preferentially adsorbed gases and vapors. This action occurs to a greater or lesser degree with all adsorbent materials resulting in the production of a desorbed odor complex atypical of the original adsorbed odor. As a result relatively large quantities of activated carbon are required in such filter devices; however, even with the larger amount of activated carbon the relatively large pressure differential problem remains.
The use of activated forms of alumina as a support in gas filters is known. Activated aluminas have been coated and or impregnated with various agents by methods of varying complexities. The principal problem in impregnating active alumina has been to economically impregnate the alumina with an effective chemical composition while maintaining the desired degree of activity and resistance to abrasion, without destruction of the impregnant material or support. Materials which are particularly difficult to impregnate into active alumina while maintaining sufficient strength and desired properties are the permanganates of alkali metals, notably potassium permanganate. However, permanganate ions have rapid reaction kinetics and a broad reaction profile for inorganic and organic moieties and therefore have long been used in odor removal in liquid scrubber applications and in dry supported forms. They also have a history of demonstrated performance for a wide range of odors. Permanganate ions are strong oxidizing agents capable of reacting with aldehydes, reduced sulfur compounds, unsaturated hydrocarbons, alcohols, phenols, amines, hydrogen sulfide, sulfur dioxide, etc. Therefore it is desirable to have a filter media with a support material impregnated with permanganate.
Early development work to produce satisfactory activated alumina forms containing an oxidizer such as permanganate generally involve pelletizing the alumina starting material, then evacuating the alumina pellets to clear the pores so that the alumina forms could be effectively impregnated with an oxidizing agent. Impregnation of the alumina forms has typically been accomplished by spraying the forms with an impregnating solution. After impregnation with the oxidizing agent, the alumina forms are dried to remove uncombined water. Among the problems encountered in attempting to adapt presently used commercial procedures for the production of permanganate impregnated alumina composites has been the difficulty to obtain the desired uniform distribution of the oxidizing agent throughout the alumina mass. Furthermore, the economical formation of filter media having a desired concentration of permanganate, a desired pellet size or shape, and a desired physical strength has remained a challenge.